Improving platinum group metal utilization for emission control catalysts through thermally–induced restructuring of core@shell nanoparticles

Alexander  Hill, University of Michigan

Introduction

The widespread adoption of automotive emissions control technologies has considerably increased the demand, cost, and scarcity of the platinum group metals (PGMs) that comprise three-way catalysts (palladium, platinum, rhodium). Consequently, there is an ever-growing interest in finding ways to reduce or better utilize PGM content to meet the significant worldwide demand of emissions control catalysts.

PGM utilization is significantly compromised in emissions control applications during prolonged periods of high temperature operation. This is due to the thermally–induced sintering of both active metal and support components, which significantly decreases the abundance of accessible active sites and inhibits access to the oxygen storage capacity (OSC) provided by reducible oxide supports such as CeO2. Here, we investigate how initial catalyst morphology influences restructuring outcomes by comparing the high temperature aging of Pd@CeO2 core@shell nanoparticles, synthesized by a scalable, one–pot synthesis approach, with Pd/CeO2 catalysts prepared by wet impregnation.[1] We show divergent restructuring outcomes that depend on the placement of the active metal.

Materials and Methods 

Core@shell catalysts were synthesized using a one-pot synthesis approach where Pd nanoparticles are encapsulated through base-catalyzed polymerization of CeO2 precursors.[2] Pd/CeO2 catalysts were prepared by incipient wetness impregnation of Pd onto previously synthesized CeO2 nanospheres. Catalysts were aged at 800 ℃ for 4 hours in 2.5% O2 (N2 balance), achieved with a temperature ramp of 2 ℃/min.

Bright field and dark field transmission electron microscopy in addition to x–ray energy dispersive spectroscopy was used to characterize the fresh and aged catalysts. Porosity characterization was performed using Brunauer–Emmett–Teller surface area analysis. X-ray photoelectron spectroscopy was performed to investigate changes in the oxidation state and coordination of Pd, and x–ray powder diffraction was used to examine changes in the crystallinity of the CeO2 shell that resulted from high temperature aging.

Carbon monoxide oxidation was used as a probe reaction to evaluate the catalytic activity prior to and post aging. The reaction mixture was composed of 1% CO, 1.5% O2 with N2 balance and sent through a 4 mm ID packed bed reactor at a rate of 200 mL/min. A downstream FTIR was used to analyze the effluent gas. T90 was used to define the light off temperature and provide a quantitative metric of catalytic efficacy.

Results and Discussion

800  aging of Pd/CeO2 catalysts results in appreciable pore closure, active metal sintering, and impeded low temperature activity. In contrast, the same aging protocol applied to the Pd@CeO2 catalysts redisperses Pd throughout the encapsulating CeO2 shell. This increases the active site dispersion from 33 to 88%. The resulting Pd clusters are small (average diameter of 1.3 nm) and are found to coordinate very strongly with the reducible CeO2 support, as confirmed by CO chemisorption, transmission electron microscopy, and x-ray energy dispersive, and photoelectron spectroscopies. The redispersion of Pd also appears to stabilize the CeO2 crystallites that comprise the shell, making them less prone to sintering. These restructuring processes appreciably increase activity, particularly in the cold start regime, as evidenced by a 45  reduction in the light–off temperature for CO oxidation. The turnover frequency for CO oxidation increases from 0.2 to 0.39 /s and the apparent activation energy decreases from 42 to 33 kJ/mol after aging. These changes in intrinsic activity suggest that the increase in Pd dispersion is not the sole reason for the increased performance in CO oxidation seen in the restructured catalysts. Subsequent studies illustrate that the restructured Pd@CeO2 catalyst exhibits enhanced accessibility to the OSC of the CeO2 support.

Significance

The findings from this study illustrate that the initial catalyst design plays a significant role in dictating the outcomes of thermally induced restructuring. The separation of active metal domains, through encapsulation in a core@shell morphology, facilitates active metal redispersion, which appreciably increases low temperature activity and access to support OSC. This suggests that the thermally–induced restructuring of core@shell catalysts may be a promising strategy of increasing the utilization of costly PGMs in emissions control applications.

REFERENCES: 

[1] Hill, A. J.; Seo, C. Y.; Chen, X.; Bhat, A.; Fisher, G. B.; Lenert, A.; Schwank, J. W. Thermally Induced Restructuring of Pd@CeO2 and Pd@SiO2 Nanoparticles as a Strategy for Enhancing Low-Temperature Catalytic Activity. ACS Catal. 2020, 10, 1731 – 1741. https://doi.org/10.1021/acscatal.9b05224.

[2] Seo, C.; Yi, E.; Nahata, M.; Laine, R. M.; Schwank, J. W. Facile, One-Pot Synthesis of Pd@CeO2 Core@shell Nanoparticles in Aqueous Environment by Controlled Hydrolysis of Metalloorganic Cerium Precursor. Mater. Lett. 2017, 206, 105 – 108. https://doi.org/10.1016/j.matlet.2017.06.114.